1. Field of the Invention
The invention relates to a color density measuring device for determining the color density of an ink layer applied to a printing material or subject, the color density measuring device having a light source for illuminating the printing material, and a sensor for receiving the light remitted from the printing material. The invention further relates to the use of such a color density measuring device on a printing apparatus for multicolor printing. Moreover, the invention also relates to a method for determining the color density of at least one of the three colors cyan, magenta or yellow, or black on a colored material.
2. Description of the Related Art
Particularly for the purpose of controlling the ink management in the case of printing units for multicolor printing, it is customary to determine the color density of an ink layer applied to the printing material. So-called reflected colored light densitometers are currently used for this purpose. This type of measurement is based on a linearity between a printed quantity of ink and optical density. The density measurement is performed using the known principle of remission measurement of body colors. An emitted measuring light beam penetrates the ink layer and is reflected at the interface between ink layer and printing material. The reflected light penetrates the ink layer once again and is received by a photoelement of the reflected colored light densitometer. Upon two-fold penetration of the ink layer, the originally white measuring light is colored and attenuated by absorption. The remission of the light is reduced in a typical wavelength region. An electric signal proportional to the light intensity is generated at the photoelement. This electric signal contains the information, resulting from the modulated reflected light, relating to type and quantity of the printed color that is used to determine the density value.
Densitometers having so-called status filters or color filters are known which can be used for separately detecting the printing inks cyan, magenta, yellow and also black. In the case of these densitometers, individual photoelements for white light are arranged downstream of a total of four individual filters each having different transmission ranges. The photocells form individual measuring channels for each of the printing inks cyan, magenta and yellow. The density of these colored inks is determined in each case in the low remission range because variations in remission take place here that are prominent and can be effectively measured. The density of cyan is therefore measured with the aid of a red filter at a wavelength of approximately 600 nm. Similarly, magenta is measured with a green filter at approximately 530 nm, and yellow is measured with a blue filter at approximately 430 nm.
The characteristic of the filters is laid down in the standards DIN 16 536-2 and ISO 5-3. Filters of status DIN E are used in Europe. With regard to the SWOP color standard, ISO T filters are customary in the USA and, to some extent, in England.
This procedure is illustrated in FIG. 1 using the example of remission curves of cyan for various color densities. The density is measured with the aid of a red filter (transmission range at a wavelength of approximately 600 nm) in the region of the largest variations.
It is an object of the invention to specify a color density measuring device for determining the color density and the use thereof, as well as a method for determining the color density which together lead to a substantial reduction in cost by comparison with known measuring devices and methods.